1. Technical Field
The present invention relates to dense wavelength division multiplexed optical networks, and in particular to reconfigurable add/drop multiplexers for use in such networks.
2. Description of the Related Art
Reconfigurable Add/Drop Multiplexers (ROADM) are an important element in the dense wavelength division multiplexed (DWDM) optical network. ROADM nodes have been deployed in long haul and metro optical networks. In long haul networks a ROADM node is typically placed every several spans, while metro optical networks require denser placement of ROADM nodes, even up to one node per span. A ROADM node allows the flexible adding and dropping of any or all DWDM channels. Multi-degree ROADM nodes (nodes with 3 degrees or higher) also provide a cross-connection function for DWDM signals among different degrees. This function is also called wavelength cross-connect (WXC).
As the traffic of the network becomes more dynamic and the network topology evolves from ring to mesh or meshed ring, the limitations of prior art ROADM nodes become more significant. Two main limitations are colored transponder assignment and directed adding. The colored transponder assignment means that each transponder corresponds to a fixed wavelength (that is, a certain color), and therefore all transponders need to be preinstalled (high capital expense) or manually provisioned during system reconfiguration and upgrade (high operation expense). Because each end of an optical connection must use the same wavelength, the statically colored designs of the prior art greatly limit the flexibility of a network.
The directed adding issue refers to the fact that the outbound direction of the added signals is limited to the same degree and cannot reach other ROADM degrees (although with an optical protection switch, it can reach 2 degrees). This prevents the signals from being routed to different optical paths and thus further limits the flexibility of the node and the network.
Several architectures have been proposed to address these issues, but all of them suffer from a lack of in-service upgradeability, as they require large port-count photonic cross-connects (PXCs). PXCs are physically large and are unsuited for use in telecommunication-grade applications. The prior art architectures further suffer weaknesses in equipment reliability and node modularity.